The present invention relates to a method for reducing the particle size of water-insoluble, polysaccharide derivatives and to aqueous colloidal dispersions of the polysaccharide derivative.
Water-insoluble, polysaccharide derivatives such as the cellulose ethers, e.g., ethyl cellulose and benzyl cellulose, are known to be particularly useful in a wide variety of applications including the preparation of molded articles and as a component in protective coatings and adhesives.
Conventionally, in the preparation of films, coatings or the like, an aqueous suspension of colloidal size particles of the polysaccharide derivative is applied to a suitable substrate and the particles subsequently fused to form the desired film or coating. In general, to prepare films or coatings having desired properties and uniformity, the particles of the polysaccharide derivative exhibit a number average particle size of less than about 100 microns. Therefore, as the polysaccharide derivatives are conventionally prepared as a powder having a relatively large particle size, e.g., number average particle size from about 1 to about 10 mm, they must often be further comminuted prior to use.
Unfortunately, conventional grinding techniques have not been found to effectively comminute the polysaccharide derivatives. Specifically, the polysaccharide derivative must be ground in an oxygen free atmosphere to prevent the possibility of explosion. In addition, the temperature incurred during grinding is often sufficient to prematurely fuse the polysaccharide derivative particles, thereby deleteriously affecting the properties of the ground material. Moreover, to obtain the desired particle size generally requires significant amounts of time and energy.
To reduce the problems associated with possible dust explosions, it has been proposed to dissolve the polysaccharide derivative in an organic liquid and to disperse the resulting solution in water. Thereafter, the organic liquid is evaporated with the resulting product being an aqueous suspension of colloidal size particles of the polysaccharide derivative. Unfortunately, dispersing the organic solution in water and thereafter evaporating the organic liquid requires undesirable amounts of time and energy. In addition, the ground polysaccharide derivative often contains undesirable amounts of residual organic liquid.
Alternatively, as disclosed in U.S. Pat. No. 4,076,935, the particle size of cellulose or a derivative thereof can be reduced by grinding the cellulose or derivative in a vibration mill in the presence of air and sufficient amounts of water to yield a product having a water content from about 2 to 10 weight percent. While this method also reduces the possibility of explosion during the grinding operation, polysaccharide derivatives having a desirably small particle size are not readily obtained without undesirable amounts of the cellulose or the cellulose derivative being fused and/or exhibiting significant viscosity losses during the grinding operation.
Therefore, in view of the stated deficiencies of the prior art methods of comminuting polysaccharide derivatives, it remains highly desirable to provide a method for effectively comminuting polysaccharide derivatives.